A respiration mode APRV (Airway Pressure Release Ventilation), which is, similarly to the CPAP mode, a mode with a continuous positive airway pressure (CPAP), is known from practice. Contrary to the CPAP respiration mode, an upper pressure Phigh is set in the APRV mode. This pressure Phigh is regularly lowered for a short duration each time to a lower pressure level Plow, this pressure reduction being called “pressure release.” The patient being respirated by means of APRV can breathe spontaneously at any time, but the pressure release is not synchronized with the patient's respiratory activity. It is typical of the APRV mode that a duration Thigh, during which the pressure Phigh is applied, is usually longer than a duration Tlow, during which the lower pressure Plow is present. FIGS. 1A and 1B show an example of a respiration in the APRV mode in a simulation.
During respiration in the APRV mode, the mean respiration pressure is maintained at a comparatively high level, which leads to improved oxygenation. The elimination of CO2 is supported by the pressure release. Due to the short duration Tlow and the short duration of the pressure release, removal of air from the lungs to the extent that alveoli could collapse and could not participate in the ventilation any longer is prevented from occurring. The duration Tlow is set such that complete breathing out is prevented from occurring. It can be recognized from FIG. 1B that the pressure release is terminated before the patient flow has risen to 0 L/minute (see the times t=5.5 sec; t=13 sec; t=21.5 sec in FIG. 1B).
A new set value for APRV ventilation, which is circumscribed as “optimal flow termination based on a percentage of peak expiratory flow,” is proposed in the patent application US 2006/0174884 A1 of Nader M. Habashi. This set value will hereinafter be called % PEF (Peak Expiratory Flow). The parameter % PEF is set in percentage of a maximum expiratory flow PEF. The pressure release is terminated when the instantaneous expiratory flow of the patient relative to the maximum expiration flow has dropped below the percentage set as % PEF (cf. FIG. 2). As a result, the algorithm automatically adapts the duration of pressure release to changes in the lungs.
However, respiration in the APRV respiration mode with the % PEF criterion, as it is described in the above-mentioned US 2006/0174884 A1, has the following problem: since the APRV mode is not synchronized with the patient, the patient can breathe spontaneously at any time, i.e., even during the pressure release phase. Due to such a spontaneous respiratory effort, the patient can generate a stronger inspiratory flow, above all in case of smaller pressure differences between Phigh and Plow, than the expiratory flow generated by the respirator. As a consequence of this, a positive (inspiratory) patient flow prevails instead of the expected expiratory flow, and the % PEF criterion cannot therefore be met or it will be met at a markedly later point in time only, namely, when the spontaneous inspiration of the patient again passes over into an expiration. Until the set % PEF criterion is met during the latter expiration, the respirator applies the low pressure Plow over an undesirably long time period. A safety means may be provided for interrupting the expiration after the end of a set maximum duration Tlow. However, this is a safety means, which cannot prevent an excessive removal of air from the lungs with the above-described problems and risks during the prolonged expiration time.